Things You'll Need
Scrub brush (a toothbrush works fine for smaller stains)
Fels Naptha soap (available in most grocery stores or online--a bar costs about a dollar)
Stain spray (I use Spray 'N Wash)
Warm or hot water
A large bucket or sink
Laundry detergent and a washing machine
Be careful treating items that are not colorfast or items made from delicate fabric. This process may fade or damage delicate clothing. Only use the warmest water recommended on the clothing tag, or item may shrink or be damaged in some other way.
Always handle cleaners and stain removers with care--especially if you have sensitive skin. You may want to wear rubber gloves and/or eye protection, just to be safe.
After washing clothes for three kids, I've finally discovered a nearly foolproof way to get those yucky yellow baby poop stains out of clothing, even when it's been sitting for weeks! It's worked for me every time. If you're worried about the colorfastness of a fabric, or using this method on delicate fabrics (which I don't recommend unless you're really desperate), test a small,unnoticeable area first.
Remove as much of the poop as possible. Rub the scrub brush across the bar of Fels Naptha soap, then scrub the soap into the stain under running water. Use as warm of water as you can (refer to the recommended washing temperature on the clothing tag). Continue rubbing Fels Naptha into the stain and scrubbing with the scrub brush until you've removed as much of the stain as you can. Try not to rinse all the Fels Naptha soap out of the fabric.
Fill a large bucket or sink with a gallon or so of hot water. Add a scoop of Oxy Clean.
Add 1/4 cup or so of washing soda to the water.
Finally, add 1/4 cup or so of stain spray to the water. Put stained items in the water and allow to soak for 2 to 3 days depending on the severity of the stain.
Dump contents of bucket into washing machine. Add laundry detergent and run through a wash cycle on warm water. Check stain (I bet it will be gone!) and if it's still there, repeat process as necessary. Do not dry items in the dryer until the stain is gone, or it will be set in. Good luck!